Jan Budroweit
©SHUTTERSTOCK.COM/ANDREY SUSLOV
Growing interest in the commercialization of space (NewSpace) is leading to increased acceptance of commercial off-the-shelf (COTS) electronics and thus enabling use in space of the latest technologies developed for terrestrial applications. This opens new capabilities for applications in space missions. RF communication via satellites has been established for decades; traditionally used for military applications, it is becoming increasingly important for commercial users. Today, global connectivity and access to the Internet has never been so important, and the demand for worldwide connectivity increases significantly each year. Space-based Internet access is especially important for areas with poor terrestrial infrastructure, such as Africa and Antarctica. There are already geostationary-Earth orbit (GEO)-stationary-located satellites available such as Inmarsat or low-Earth orbit (LEO) satellite constellations like Iridium, which provide almost-global access to the Internet, but they are either very expensive or have low data rates and long latencies, so they are not very attractive. New solutions with high data rates, low latencies, and affordable fees are currently in development, such as StarLink, with its giga constellation [1], [2], and Amazon’s Project Kuiper [3], [4], [5]. So it is true: there really has been a change in the market with the NewSpace Era. With NewSpace, or in other words, the commercialization of space, traditional space missions with high-quality assurance requirements, very long design and manufacturing times, and extraordinary costs may become obsolete at a certain point, at least when it comes to commercial services like satellite-based Internet access. To decrease the lead time and costs of such missions, satellite designers and manufacturers need to rely on commercially available electronics, namely, COTS devices, which are usually much cheaper and have better performance and shorter lead times compared to space-qualified parts. In terms of performance, COTS devices have great benefits over space-qualified parts simply because the design, development, and qualification processes for space parts are very complex and take years before they are qualified and available for the market. It is likely that state-of-the-art space-qualified parts are 10 years behind what we currently have available for terrestrial applications, such as for the automotive or industrial markets [6]. One obvious reason is certainly that electronic parts manufacturers develop products for certain markets, and space is yet not one of the biggest or most attractive markets. But this has already started changing and big players such as Texas Instruments and Analog Devices are moving forward with the NewSpace wave and establishing ever-more products for commercial space applications. Truly, using state-of-the art electronics is not only of interest for the space-based Internet, there are also other space-related applications that can clearly benefit from using the latest technologies.
Over the past few years, an increasing number of applications for RF and microwave bands have been established. Even though those applications are reserved for specific frequency bands and are regulated by national and international organizations, such as the International Telecommunications Union (ITU), interference and jamming are increasingly becoming urgent issues. Specifically, scientific experiments in space, such as radiometer applications, suffer from interferences like those described in [7], [8], and [9]. The location of such jammers is quite challenging, especially on Earth, where observation areas are limited. Thus, space solutions capable of monitoring large regions for RF interference (RFI) would be meaningful.
When it comes to space applications, radio systems play an essential role. They are used to establish a communication link between the spacecraft (i.e., satellite) and the ground station, either to control a vehicle or to receive telemetry or payload data. Furthermore, many scientific applications are of interest for space; for example, receiving RF data from bands of interest, the tracking aircraft in the L-band at 1,090 MHz, and general RF geolocation at multiple frequency bands. In the following section, the state of the art for radio systems in space applications is described first, and then the focus turns to recent applications for RF-based geolocation from space.
Radio systems have been established in space for many years. Traditionally, those radio systems were made to be very reliable and thus designed discretely by selecting specific RF components and performing the signal processing (e.g., demodulation) in an analog way. Modern radio architectures consist of analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), which in fact require digital signal processing of the RF data, which we call software-defined radio (SDR). SDRs were first used in the early 1990s [10], [11], [12]. Besides the fact that digital signal processing reduces the number of components in a radio system, there is another great advantage to using SDRs in space applications: they allow reconfiguration and modification of digital signal processing, even if the spacecraft is already launched into space and there is no hardware revision required [13], [14], [15], [16], [17]. SDRs were usually established in a superheterodyne or synchrodyne architecture and specifically designed for one application (e.g., the reception of telecommands and the transmission of the telemetry of a spacecraft in a dedicated frequency band, for example, the S-band).
However, although SDR technologies may overcome the problems of hardware-revision changes and the upload or modification of digital signal processing, SDRs are still capable of only a single application.
With increased numbers of satellites launched, the use of COTS devices is increasingly popular. Clearly, the use of COTS enables new possibilities and makes SDRs much more efficient and affordable, but on the other hand, the reliability of such systems becomes crucial due to the fact that COTS devices are not designed and fabricated for the harsh environment of space [18]. Specifically, radiation effects and commercial development’s less-stringent requirements for traceability and quality control leads large satellite manufacturers to still use space-qualified devices, which are in fact much more expensive and, from a technical and performance point of view, far behind what is state of the art for terrestrial applications.
Thus, there is a large gap between traditional space-mission approaches and the more risk-friendly terrestrial community, many of whom come from academia with very limited budgets. CubeSats missions, which were initially university projects, have now become very popular, with many manufacturers with space-flight heritage available in the market [19], [20]. Even though bigger companies and space agencies have not yet moved toward CubeSat-related technologies, the ideas of miniaturization and cost reduction are now of great interest in the space community [21], [22]. Today, offering commercial services throughout low-cost missions is a leading differentiator and can be seen in companies like StarLink or in Amazon’s Project Kuiper, which already launches thousands of satellites, with tens of thousands planned. This approach is currently leading toward a trend of using COTS devices, even in critical space missions.
For SDRs, these types of developments can also be seen in state-of-the art technologies like RF integrated circuits (RFICs) such as the AD9361 from Analog Devices, which are usually designed for terrestrial mobile services like 4G and 5G. With such technologies, SDRs can also be reconfigured in their RF properties and are no longer limited to digital signal processing [23], [24], [25]. With the ability to control RF front-end properties with software, multiple applications can be conducted using a single radio platform. This will significantly reduce the required mass and size of a radio platform and allow integration and operation, even on smaller satellites. Another benefit of software-controlled front-end circuits is the observation of different frequency bands, which allows RF-based geolocation of radio signals and interference with a single radio unit, which is described in the next section.
As was pointed out in the previous section, with the capabilities of new SDRs, implementation of RF-based geolocation on small satellites has become feasible. Such systems are commonly used to switch among frequency bands of interest (see Table 1).
Table 1. Potential applications and their frequency bands for RF-based geolocation from space.
In Table 1, just a few potential applications are listed where signal monitoring is of interest and which are already established, as described further in this section. Additional areas of interest are spectrum mapping to ensure and increase efficiency, detect interference or jammer activities in certain frequency bands, and for space situational awareness. They are not only of interest for governmental uses but also for commercial services.
As mentioned previously, scientific space missions with radiometer applications suffer from RFI. As an example, the European Space Agency’s Soil Moisture and Ocean Salinity mission, which was launched in 2009, has been strongly impacted by radio interference during its 10 years of operation [8]. The radiometer operates in the 1,400–1,427-MHz frequency band, which is allocated to the Earth-exploration satellite service, where no assigned transmission is allowed. However, to date, the instruments have recorded more than 500 active RFI sources distributed all over the world. The other missions that have reported RFI from different frequency bands (from the L-band up to the K-band) are the Japanese AMSR2, NASA’s Global Precipitation Measurement (GPM) Microwave Imager (GMI), and the U.S. Navy WindSat [7], [9].
With increased services on the ground and also in space, where frequency regulation is still challenging (e.g., intersatellite links), the issues with RFIs will increase. This trend was already demonstrated by Draper and Matthaeis in [7], where RFIs to radiometer missions over the last 15–30 years were evaluated. The ITU is already addressing these issues and working on IEEE standards to assess interference on remote sensing frequency bands [9].
RF sensing systems in space applications are already established and available as commercial services, including the Aurora Insight [26] and HawkEye 360 (HE360) Pathfinder missions, which successfully placed three microsatellites in LEO. They have already demonstrated their capabilities for RF spectrum monitoring and RF-based geolocation from space [27], [28], [29]. In [30], the importance of RF sensing systems is summarized, and further (upcoming) space missions that are equipped with appropriate RF systems are presented. To further investigate the state of the art of existing RF-based geolocation satellite missions, and to point out their weaknesses and how newer technologies like RF systems on chip (RFSoCs) (described in this section) and mixed-signal front ends (MxFEs) can improve performance, the HE360 satellite architecture is discussed, specifically its instruments, which allow spectrum monitoring of a variety of frequency bands.
As previously mentioned, HE360’s Pathfinder mission is a cluster of three nanosatellites with a mass of approximately 15 kg that was launched in late 2018. An illustration of one HE360 with its large number of different antennas is depicted in Figure 1. Its primary objective is to enable precise spaceborne geolocation of terrestrial and aerial RF emitters, ranging from logistics monitoring, to tracking of aircraft and marine vessels, to emergency response [27], [28]. A geographical illustration of already-captured RF signals of the HE360 is presented in Figure 2.
Figure 1. An illustration of the HE360 Pathfinder satellite (Source: [28].)
Figure 2. The geographical illustration of an RF signal received by the HE360 Pathfinder satellite. (Source: HE360.)
To detect different services and RF signals, HE360 satellites are equipped with an SDR payload that is based on Analog Devices’ AD9361, which was mentioned in the previous section. That device enables software-controlled coverage from 70 MHz to 6 GHz, covering an RF bandwidth of up to 56 MHz per channel, without hardware modification on the SDR. The HE360’s SDR is based on a direct downconversion architecture, meaning that the carrier frequency is selected by software and configured to the AD9361, which downconverts it to the baseband. Thus, the ADC can directly convert the baseband signal to the digital domain, even when complex filtering or multiple downconversion stages are required. This truly has great benefits in terms of size, weight, and power (SwaP) compared to a traditional superheterodyne receiver architecture [31].
As the satellite’s orbit has an altitude of 575 km (a sun-synchronous orbit), the signals received from different frequency bands are expected to be very weak and need to be amplified and preconditioned before the AD9361 is capable of efficiently converting the RF signal into the digital domain for further data processing. Thus, the HE360 has application (or frequency band)-specific front-end circuits that are placed in front of the SDR, which is usually equipped with dedicated filters and low-noise amplifiers. The front-end circuits are individually switchable to the SDR input and are attached to a corresponding antenna. Thus, the capabilities of the SDR are limited to one application at a time. A simplified block diagram of the SDR payload is presented in Figure 3.
Figure 3. A simplified block diagram of the HE360 Pathfinder SDR payload, according to [28]. ANT: antenna; LNA: low-noise amplifier; RFIC: RF integrated circuit.
Another potential limitation of the SDR payload is the AD9361’s RF bandwidth of 56 MHz. Although for many lower-frequency applications no issue is likely to occur as the desired observation band is fairly narrow (e.g., the Automatic Dependent Surveillance-Broadcast for aircraft tracking at 1,090 MHz is ∼1 MHz), for higher frequencies such as X band or Ka-band, higher bandwidth capabilities may be required that are not covered by that RFIC device. Furthermore, any new application or frequency band of interest needs to have dedicated front-end circuits, which require further hardware revisions, currently impossible for in-space satellites.
Even though the HE360 and its counterparts have already proven their ability to detect and geolocate RFI sources, they do have some weaknesses, which can be improved using the latest technologies such as RFSoC and MxFE, both of which are introduced in the next section.
In the previous section, the latest SDR platforms for space applications that allow a software-based reconfiguration of RF front-end properties and cover a frequency range of 70 MHz to 6 GHz with a capable RF bandwidth of 56 MHz were discussed. Such SDRs are based on direct downconversion architecture, which is already quite beneficial in terms of SwaP compared to the well-known superheterodyne receiver architecture; we have also seen some limitations of these systems. Due to the fact that the RF bandwidth is limited, we need to focus on newer technologies that overcome these limitations. So-called RFSoCs and MxFE technologies, devices that consist of high-speed ADCs and DACs that allow direct sampling radio architectures, could become a possible solution.
Direct sampling of RF signals has clear advantages as the implementation is very easy and does not require any specific mixers, and the number of RF front-end components (e.g., filters and mixers) is greatly reduced. Depending on the ADC/DAC performance and sampling-rate features (speed), extremely high bandwidth capabilities on the order of >6 GHz can be enabled, which is not achievable with the previously described radio architecture. Furthermore, distortions injected by mixers and amplifiers are avoided, and thus, no additional noise or electromagnetic compatibility issues are expected. In such a direct sampling implementation, the gain is at the operating frequency band, and therefore, a careful layout design is required if high gain in a receiver is desired. As a matter of fact, extreme high sampling rates of ADCs and DACs are required. Currently available ADCs and DACs allow operation in lower-frequency bands up to 6 GHz, but newer technologies will be available in the future that enable operation in higher-frequency bands, such as the X band. For direct sampling, jitter of the sampling clock or phase noise becomes a concern, which could negatively affect signal-to-noise ratio of the ADC. The simplified schematic of a direct sampling architecture is presented in Figure 4.
Figure 4. The schematic of a direct sampling receiver architecture. LNA: low-noise amplifier.
Additional challenges related to those architectures include the usually very power-hungry ADCs and DACs, which consume far more power compared to the previously described radio designs. Also, as sampling rates on the order of giga samples per second (GSPS) are used, interface requirements for signal processing need to be considered as extremely large amounts of data need to be transferred and processed. A list of challenges and advantages with direct sampling radio architectures is given in Table 2.
Table 2. Advantages and challenges for direct sampling architectures.
Moreover, the available bandwidth for such systems unveils limits from other radio/receiver architecture designs as ultrahigh-speed ADCs and DACs are used that support >6 GHz RF bandwidth processing. In the following, an overview is provided of currently available solutions such as Xilinx’s RFSoC and Analog Devices’ MxFE, which are designed to implement such direct sampling architectures efficiently as they already integrate a lot of functionality in a single chip, enabling small radio system designs.
RFSoCs combine well-known SoCs featuring integrated, programmable logics and digital processors with high-speed ADCs and DACs in a single device. An example is the Xilinx Zynq UltraScale+ RFSoC family [32], [33], as depicted in Figure 5.
Figure 5. Zynq UltraScale+ RFSoC. (Source [34].) DDC: digital downconversion; Rx: receiver; Tx: transmitter FPGA: field-programmable gate array; DUC: digital upconversion; CPRI: common public radio interface; GTY: gigabit transceiver; GE: gigabit ethernet.
In this family, the largest devices can provide up to 16 channels for RF signal reception and up to 16 channels for RF transmission. The ADCs and DACs have a resolution of 14 bits and can be operated with a maximum sampling rate of 8 GSPS for the ADC and up to 10 GSPS for the DAC. The newest generation can provide up to the 7.125-GHz RF bandwidth. In fact, the number of available channels and the maximum operating speed depend on each other. The major purpose of the RFSoC is obviously for mobile services such as 5G. With the availability of multiple RF channels that can be operated individually, massive multiple input/multiple output (MIMO) applications are feasible. Additionally, such technology becomes of great interest for digital phased arrays and radar applications. Further features of the Xilinx RFSoC are digital predistortion, crest factor reduction, and channel filtering, which are highlighted and described in its corresponding manual and data sheet. The key feature of the RFSoC is its high integrity as processing of the ADC/DAC data takes place in the same device as the programmable logic and processing system. As mentioned previously, direct sampling radio architectures are generally very power hungry. The RFSoC requires only 8–12 W for the ADC and DAC parts, not including the processing system and the programmable logic. Thus, thermal constraints need to be considered by the design of such radio systems.
MxFEs are, in principle, devices that support direct sampling architectures to enable SDRs with bandwidth capabilities up to 7.5 GHz. Like the previously described RFSoC, an MxFE consists of multiple high-speed ADCs and DACs that are able to capture the straight RF signal without any RF preprocessing (i.e., downconversion or filtering). The schematic overview of an MxFE is presented in Figure 6. The MxFE presented here features four 16-bit 12-GSPS RF DACs and four 12-bit 4-GSPS RF ADC cores. RF-specific signal processing such as digital downconversion or digital upconversion takes place in the device to optimize performance. Furthermore, the device has programmable filter features and is capable of protecting power amplifiers from overranging. The device comes with 24.75 Gb/s/lane JESD204C interfaces to exchange data with the baseband processing unit, which is basically a field-programmable gate array (FPGA) device. This required FPGA is already integrated in the RFSoC, as described previously. Additional information is available from the AD9081 data sheet [35]. The advantage of the MxFE is its integrated on-chip digital signal processing and the clock/phase-locked loop that allows multichip synchronization. As a result, multiple devices can be synchronized and are then also of high interest for MIMO, phased-array/beamforming, and radar applications.
Figure 6. A schematic overview of the MxFE technology. DDC: digital downconversion; DUC: digital upconversion; PA: power amplifier; PLL: phase-locked loop; Prog.: programmable.
As was pointed out in this article, currently available direct downconversion architectures for SDRs have great advantages in terms of reconfiguration capabilities, and wide coverage from 70 MHz to 6 GHz. Thus, several frequency bands can be observed in terms of RF-based geolocation and spectrum monitoring. However, limitations are the quite-narrow RF bandwidth of such SDRs of 56 MHz and the required specific RF front ends that are multiplexed in front of the receiver’s input. In the presented example of the HE360 Pathfinder, only one frequency band can be observed at a time and, broad coverage, as we know from spectrum analyzer measurement equipment, is not feasible.
The technologies introduced in this article for direct-conversion radio architectures can overcome those limitations. As those devices are capable of processing an effective RF bandwidth of >6 GHz, real spectrum monitoring can be achieved. Furthermore, because the devices consist of several (up to 16) ADC and DAC channels, multiple front-end circuits and antennas can be attached to the device, and simultaneous data processing becomes feasible. This is also of great interest for MIMO and radar applications in space with active beamforming intentions.
Although we already know that currently used SDR architectures are operational on small satellites and have great advantages in terms of SWaP, the RFSoC and MxFE technologies proposed here can significantly improve this SWaP figure. For space missions, such technologies could be realized in small form factors and lightweight designs. Specifically, when it comes to the use of channel capacities, MxFEs and RFSoCs require a much smaller size in the system design than do RFIC technologies, where multiple devices need to be integrated and synchronized.
That greatly affects the overall cost of a space mission, and not only the launch costs, which are typically in the range of >US$10,000 per kilogram dry mass (even though launch costs will rapidly decrease with the advent of space-launch operators like SpaceX [36], [37], [38]). Moreover, they will have a great advantage in terms of reducing the effort for analysis, integration, and testing. As a result, a shorter development and manufacturing time for the satellite mission can be achieved. Additionally, smaller radio designs can be more easily used in smaller satellite structures (e.g., CubeSats) where more launch opportunities are available and which also positively affects the overall mission cost.
There are many other interesting applications that are enabled by these technologies, which could be of great interest for space missions. As the market accelerates its trend toward smaller satellite systems, these RFSoC and MxFE technologies will fit the constraints and requirements of those satellites.
This article presented state-of-the-art SDR architecture applications, with a specific focus on RF-based geolocation from space. Today, the most typical implementation of radio systems in space applications relies on devices deemed reliable for space based on a rigorous, expensive, and time-consuming process, which limits the possibilities that could be achieved with the latest technologies that are designed for terrestrial applications, such as for mobile services. Direct-conversion SDRs based on RFSoC or MxFE technologies could dramatically reduce the required size for such a radio system and would enable their use on even smaller satellites in the range of tens of kilograms. Specifically, for RF-based geolocation in RFIs, such technologies are of great interest, but also MIMO and radar applications with active phased-array systems, such RFSoCs and MxFEs, can be a key technology due to their high number of available RF channels, small size, and feature of software-based reconfiguration.
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Digital Object Identifier 10.1109/MMM.2022.3217988